The invention concerns a method of modulating the exposure of photographic copies where the original to be copied is scanned point by point photoelectrically in order to derive density values for the scanned points and, from the values to derive a basic density correction factor to be used for modulation of the overall density of the copy to be prepared.
Experience has shown that various originals to be copied, usually negatives, although being generally similar in density structure, will often require very dissimilar treatment when being reproduced because of the subject portrayed in the original, namely whether the primary subject (for example one or more persons) is located within the light or dark area of the image.
If a photograph is taken by means of a flash, the important portion of the picture will usually be found within the dense portions of the negative, and will often form its densest portion. For example, in the case of photographs taken of persons the important portion, the face of the subject, will be the more dense portions because the face of such persons is being directly illuminated by the flash. It is important in these circumstances to reproduce the dense portions of the negative in a proper manner when making a copy.
On the other hand, when a street scene is being photographed in a narrow, shady street, the important subject of the picture will usually be located within the shady sections, that is within the areas of low density of the negative, while objects producing a high density of the negative, for example a portion of the sky, by chance visible at the fringe of the picture, are unimportant in this specific case. In order to reproduce properly the area containing the primary subject, a negative portraying a subject such as a street scene must be exposed to a much lesser degree than a photograph of approximately identical overall density but taken by use of a flash. Similar conditions exist in the case of snow scenes, scenes containing extensive sections of sky and exposures taken against the sun. In the case of known exposure control methods, the copies produced from such negatives are usually too dark.
In order to reduce the rate of rejects, already very low due to the presently used automatic copying systems, it is imperative to differentiate the originals to be copied in accordance with their subjects with the aid of suitable criteria and to process these originals in appropriate and diverse manners. There are several proposals known which have a similar aim. However, although these arrangements do improve the quality to a certain degree, they do not solve, or at least only partially solve, the problem discussed above. For example, German Pat. No. 10 42 374 discloses a copying method where the density of the original is scanned point by point in its three primary colors to obtain discrete density values. By weighted approximation of the mean, area densities are acquired from the discrete density values and the area densities are then used to arrive at an optical filter combination for the exposure of the material to be copied, utilizing integral density measurements. The weighted approximation to the mean gives greater weight to the density values obtained from the scanning points of the central area of the original than to the values obtained from the marginal areas of the original. This known method makes use of the empirical fact that the primary subject of the picture will be usually found at its center, and by a suitable selection of the weight-coefficients it is possible to improve the quality of copy to a certain extent. However, this method lacks sufficient sensitivity required to produce satisfactory copies in the above-discussed borderline cases.
It is further known in view of German published applications Nos. 17 72 475 and 21 43 023 to subdivide the image of the original to be copied into one single, or respectively halved, perimeter and a multitude of small partial central areas, to accomplish measurements separately of each individual area of the original and to carry out an exposure classification into well known categories on the basis of the individual test results. In the case of the method proposed by German patent application No. 17 72 475 only two values are used generally for this classification, namely the mean density of the perimeter and the maximum density of the partial central areas. In the case of the German patent application No. 21 43 023 the classification method has been refined by examining the means densities of the two half perimeters separately. Although the percentage of acceptable prints obtained utilizing these two methods is relatively high compared to integral density measurement or the method described in German Pat. No. 17 72 475, they do not meet all the requirements encountered in practice due to the insufficient number of parameters used for the classification.
Published German patent application No. 21 42 176 discloses a method for an exposure control where the density of the original to be copied is checked point by point and where density values which are derived from a specific area of the original are utilized primarily in order to determine the exposure characteristics. The specific areas are selected individually, varying from one specimen to another. The selected area comprises the important and significant area of each picture. The exposure characteristics are obtained by the evaluation of the mean density, the maximum density and the mean contrast between the scanned points of the selected area and, if necessary, the mean density of the entire original to be copied. This copying method makes it possible by a suitable selection of the specific area to be scanned to adjust the exposure of the original in such manner that it will meet all requirements necessary to emphasize correctly the main subject of each original. However, the specific scanning area must be selected visually, and this method is therefore not suitable for an automated system. If, on the other hand, the specific scanning area is fixedly set for all originals to be copied, as proposed in the published German patent application No. 21 42 176, for example, within the central area of the original, it will be possible to avoid a visually performed processing step but the parameters used by this method for the computation of the exposure characteristics will then not suffice in all instances for the production of qualitatively satisfactory copies.
A significant improvement of the exposure modulation, and as a result thereof a substantial reduction in the rate of rejects, is accomplished by a method described in U.S. Pat. No. 4,092,067 where the original to be copied, after point by point scanning, is divided into concentric zones. Intermediate values are derived for each zone from the density values of scanning points located within the zone and from the density value of one reference picture point applicable to all zones, based on specifications which vary for each zone. From these intermediate values there is then derived a final value which has a decisive influence on the setting of the general exposure. This division of the original into several zones and the individual evaluation of the density values within these zones allows a more precise determination of the density distribution within the original to be copied and a more precise exposure of the area containing the main subject.